Typical master cylinders in use in automotive vehicles for many years were arranged with one piston in a housing bore, the piston being movable to pressurize brake fluid to operate the vehicle brakes. Similar master cylinders have been used for other purposes as well, such as hydraulic clutch operators. Such master cylinders are still used for braking some farm machinery, bicycles, small off-the-road vehicles, and the like. Later developments in the automotive vehicle brake field led to the adoption of the dual or tandem type master cylinder; that is, they have two pressurizing chambers in a common bore having a closed end and an open end, with brake fluid being pressurized by a primary piston and a secondary piston arranged in tandem in the bore. One of the chambers is typically connected to one brake circuit, such as one containing the vehicle front brakes, and the other chamber is typically connected to another brake circuit containing the vehicle rear brakes. At times other circuit arrangements, and even additional pressurizing chambers, are used.
Somewhat like the earlier noted single piston master cylinders, a dual master cylinder usually has a pair of holes in the main body adjacent each of the primary and secondary piston seals. The holes are connected with the master cylinder reservoir and extend to the bore. One of the holes is a compensating port located behind the seal of the associated piston. The other hole is a bypass hole positioned in front of the seal of the associated piston. When the master cylinder is actuated, the seals move across their respective bypass holes so that these holes are no longer fluidly connected with the pressurizing chambers, and the brake fluid in those chambers and the brake circuits connected with them can be pressurized to actuate the brakes. These seals are usually cup-like or V-block seals with lips which are subject to seal extrusion into the bypass holes under some conditions, resulting in damage to the seal and loss of full functioning pressure generation for the brake circuit associated with the damaged seal. In normal service braking operation, there is little pressure in the pressurizing chambers until the bypass holes are closed, so extrusion into those holes is minimized. However, with the advent of systems variously referred to as anti-lock, anti-skid, wheel lock control, or anti-block systems the modulation of brake pressures in the brake circuits when that type system is operating can cause rapid travel of the piston seals, and particularly the secondary piston seal, back and forth across the associated bypass hole. The front bypass hole has been found to contribute to most of the seal damage in a dual master cylinder under such conditions. That seal is the bypass hole associated with the forward pressurizing chamber in the master cylinder bore, which is commonly connected to the vehicle rear wheel brakes.
Some proposals have been made to include compensation valves in the piston or pistons. While such valves have been found to function, they require additional parts and assembly.